In the field of radio receivers, there is a continuing effort to minimize the amount of tuned circuitry used. By reducing the number of tuned circuits, larger portions of the receiver may be integrated, resulting in smaller, and often less expensive, devices. This effort has resulted in widespread interest in homodyne receivers (also known as direct-conversion receivers) and low intermediate frequency (low-IF), or near-zero intermediate frequency (near-zero IF) receivers.
A well-known and common deficiency of some prior art homodyne and low-IF receivers is susceptibility to strong interfering signals. A typical front-end circuit for a radio receiver includes a filter just after the antenna input, with a bandwidth that is often significantly larger than the signal bandwidth for a given signal of interest. As a result, the signal admitted by the antenna bandpass filter may comprise one or more unwanted signals as well as the wanted signal. These unwanted signals may generate intermodulation products, among themselves and with local oscillator leakage signals appearing at the receiver input, due to square-law and higher-order distortion terms in the receiver's radio frequency (RF) circuitry. These intermodulation products may produce corrupting interference in the complex baseband signals.
Those skilled in the art will appreciate that potentially interfering signals may appear at the receiver across a spectrum extending over the total bandwidth of the receiver's RF filter or filters. Second-order (and various higher-order) intermodulation products from these signals may thus overlap the desired downconverted signal when the intermediate frequency is less than the antenna bandwidth. In the case of a homodyne or zero-IF receiver, these interfering signals may manifest themselves as a varying DC offset, which is not easily compensated by the various means commonly employed to compensate a constant DC offset. A varying DC offset is most pronounced when interfering signals are amplitude modulated, or of a bursty type, such as with time-domain multiple access (TDMA) transmissions.
The following patents issued to one of the present inventors disclose compensation of DC offset in homodyne receivers, as well as addressing other practical deficiencies such as slope and other slow drifts: U.S. Pat. No. 5,241,702 to Dent, issued Aug. 31, 1993, entitled “DC Offset Compensation in a Radio Receiver”; U.S. Pat. No. 5,568,520 to Lindquist and Dent, issued Oct. 22, 1996, entitled “Slope, Drift and Offset Compensation in Zero-IF receivers”; U.S. Pat. No. 5,712,637, issued Jan. 27, 1998, a divisional of the above '520 patent; and U.S. Pat. No. 6,473,471, issued Oct. 29, 2002, also a divisional of the above.
Various other patents disclose compensation techniques for DC offsets, including varying DC offsets, in a homodyne receiver. These patents include several issued to Lindoff et al.: U.S. Pat. No. 6,370,205 entitled “Method and Apparatus for Performing DC-Offset Compensation in a Radio Receiver,” issued Apr. 9, 2002; U.S. Pat. No. 6,449,320 entitled “Equalization with DC Offset compensation,” issued Sep. 10, 2002; and U.S. Pat. No. 7,046,720, entitled “System and Method for DC Offset Compensation in a WCDMA Receiver,” issued May 16, 2006.
In addition, U.S. Pat. No. 5,749,051, issued to current applicant Dent on May 5, 1998 and entitled “Compensation for Second Order Intermodulation in a Homodyne Receiver,” discloses compensating varying DC offsets caused by strong signals in a homodyne receiver.
All the above mentioned patents are hereby incorporated by reference herein.
Related problems due to strong interfering signals have also been found to apply to non-homodyne, low-IF receivers in which the intermediate frequency is non-zero, but still lower than the total antenna filter bandwidth. In these low-IF receivers, it is still possible for two strong interfering signals within the RF bandwidth of the antenna bandpass filter to produce intermodulation products that spectrally overlap the desired IF signal. These interfering intermodulation products include second-order intermodulation products (or, more generally, even-order products), which arise due to the square-law term in the polynomial expansion of an RF circuit's non-linear transfer function. As is well-known, the square-law term may also be reduced by employing balanced, i.e. push-pull, circuit structures. However, another mechanism that can produce interference is second-order intermodulation between one or more strong received signals, which then proceeds to modulate a local oscillator leakage signal. Local oscillator leakage in RF circuitry is a prime source of DC offset in homodyne receivers in which the local oscillator is directly on the wanted signal frequency. In low-IF receivers, strong interfering signals can effectively modulate the local oscillator leakage signal, producing spectral components that are downconverted to the intermediate frequency.
Interference from this mechanism is proportional to the magnitude of the cubic term in the transfer function non-linearity, which is not reduced by employing balanced structures, but is still a function of second-order intermodulation between the external signals. In effect, one or more strong signals inter-modulate using two of the cubic term's powers, the result of which is transferred to own local oscillator leakage via the third power. Both direct second-order intermodulation and the latter mechanism produce interference proportional to second-order intermodulation between external signals.
Although various solutions have been proposed for eliminating or reducing DC-offset problems in homodyne receiver, including those disclosed in the aforementioned U.S. Pat. No. 5,749,051 (hereinafter referred to as “the '051 patent”), further improvements are required to suppress strong signal interference arising through non-linearities in radio receivers using non-zero intermediate frequencies.